This invention relates to lighting fixtures and more particularly to lighting fixtures of the luminaire outdoor type with reflecting systems designed to produce an even distribution of light over a broad illuminated area without glare.
The efficiency of lighting fixtures is popularly measured by the lamp lumens emitted per unit of power consumption. However, this is an inaccurate measurement of the actual efficiency of the lighting fixture in achieving its primary function of providing visibility. The only true measurement of efficiency of a fixture is the level of visibility that it produces relative to the amount of energy it consumes. Foot candle levels, while being important, are hardly an accurate indication of visibility of an illuminated area. Glare from a conventional lighting fixture can cut visibility as much as 40 percent. In terms of energy consumption, this means that it is possible to produce the same level of visibility in glare-free luminaires with 250 watt lamps as with an equal number of 400 watt lamps in high-glare units. Another factor of prime importance to the degree of visibility achieved by a lighting fixture is the uniformity of illumination provided by the fixture. The reason uniformity of illumination is important to visibility is that a given area illuminated with a given low intensity adjacent to an area illuminated with high intensity will be perceived as being much darker and much less visible to the human eye than if the given area and the areas adjacent thereto are uniformly illuminated with the same given low intensity. The human eye through the opening and closing of the iris can adjust to different levels of uniform illumination across an extremely wide range. It cannot, however, account for differences in nonuniformity of light on an illuminated surface. Similarly, it cannot adjust to glare. For these reasons, a lighting fixture which provides uniform illumination without glare can much more efficiently achieve the primary function of providing visibility than a lighting fixture which does not provide uniform illumination or which provides illumination with glare.
In order to achieve uniform illumination without glare, the inventor of the present invention developed the lighting fixtures disclosed in U.S. Pat. No. 3,836,767 and in U.S. Pat. No. 3,651,320. Both of these fixtures achieve the effect of producing uniform illumination over the illuminated area and a cutoff of light rays projected above a selected cutoff angle to eliminate glare. The present invention provides another fixture which more efficiently provides uniform illumination without glare. Moreover, the lighting fixture of the present invention provides the glare-free uniform illumination with an entirely different kind of reflector system than the fixtures of the two prior patents mentioned above and, therefore, presents an entirely different appearance. The fixture thus provides the architect with another choice and additional variety of lighting fixtures to achieve the desired effect of glare-free uniform illumination.